Sulfonate-sulfur utilization involves a portion of the assimilatory sulfate reduction pathway in Escherichia coli.

Strains of Escherichia coli lacking serine transacetylase or a positive regulator (Cys B protein) of the assimilatory sulfate reduction (ASR) pathway were unable to assimilate sulfonate-S, while single mutants in O-acetyl-L-serine sulfhydrylase (either 'A' or 'B') were able to do so. Mutants unable to reduce sulfate to sulfite were nonetheless able to form and accumulate sulfide and then cysteine from sulfonates, while strains lacking sulfite reductase were not. Thus terminal portions of the ASR pathway are involved in reduction of sulfonate-S to that of cysteine. E. coli K-12 formed cysteine more slowly, and accumulated lesser amounts of it with sulfonate-sulfur than it did from either sulfate or sulfite. These observations are consistent with our earlier report that sulfate is the preferred sulfur source when present simultaneously with a sulfonate.

Sulfonate-sulfur assimilation by yeasts resembles that of bacteria.

Three sulfonates were tested for their ability to serve as nutrients for Hansenula wingei, Rhodotorula glutinis, Trigonopsis variabilis and Saccharomyces cerevisiae. Cysteate, taurine and isethionate, under aerobic conditions, could be utilized as sources of sulfur, although in some instances final cell yields were less than those obtained with an equimolar amount of sulfate-sulfur. Sulfonate assimilation by S. cerevisiae resembled that of bacteria (reported earlier by us) in several aspects: first, sulfate-S was used in preference to that of sulfonate, when both were present; second, mutants unable to use sulfate as a source of sulfur because of deficiencies in ATP sulfurylase, adenylylsulfate kinase (APS kinase) or PAPS reductase were able to utilize sulfonates; and third, mutants deficient in sulfite reductase were unable to utilize sulfonates.

Comparative aspects of utilization of sulfonate and other sulfur sources by Escherichia coli K12.

Selected biochemical features of sulfonate assimilation in Escherichia coli K-12 were studied in detail. Competition between sulfonate-sulfur and sulfur sources with different oxidation states, such as cysteine, sulfite and sulfate, was examined. The ability of the enzyme sulfite reductase to attack the C-S linkage of sulfonates was directly examined. Intact cells formed sulfite from sulfonate-sulfur. In cysteine-grown cells, when cysteine was present with either cysteate or sulfate, assimilation of both of the more oxidized sulfur sources was substantially inhibited. In contrast, none of three sulfonates had a competitive effect on sulfate assimilation. In studies of competition between different sulfonates, the presence of taurine resulted in a decrease in cysteate uptake by one-half, while in the presence of isethionate, cysteate uptake was almost completely inhibited. In sulfite-grown cells, sulfonates had no competitive effect on sulfite utilization. An E. coli mutant lacking sulfite reductase and unable to utilize isethionate as the sole source of sulfur formed significant amounts of sulfite from isethionate. In cell extracts, sulfite reductase itself did not utilize sulfonate-sulfur as an electron acceptor. These findings indicate that sulfonate utilization may share some intermediates (e.g., sulfite) and regulatory features (repression by cysteine) of the assimilatory sulfate reductive pathway, but sulfonates do not exert regulatory effects on sulfate utilization. Other results suggest that unrecognized aspects of sulfonate metabolism, such as specific transport mechanisms for sulfonates and different regulatory features, may exist.

Sulphonate utilization by enteric bacteria.

A variety of sulphonates were tested for their ability to serve as nutrients for Escherichia coli, Enterobacter aerogenes and Serratia marcescens. Cysteate, taurine and isethionate could not serve as sole sources of carbon and energy but, under aerobic conditions, could be utilized as sources of sulphur. Both sulphate and sulphonate supported equivalent cell yields, but the generation times varied with the sulphonate being metabolized. The sulphonate-S of HEPES buffer, dodecane sulphonate and methane sulphonate was also utilized by some strains, whereas the sulphonate-S of taurocholate was not. None of the sulphonates tested served as a sulphur source for growth under anaerobic conditions. Sulphonate utilization appears to be a constitutive trait; surprisingly, however, cells of E. coli and Ent. aerogenes utilized sulphate-S in preference to that of sulphonate, when both were present. E. coli mutants unable to use sulphate as a source of sulphur because of deficiencies in sulphate permease, ATP sulphurylase, adenylylsulphate kinase (APS kinase) or glutaredoxin and thioredoxin were able to utilize sulphonates; hence sulphate is not an obligatory intermediate in sulphonate utilization. However, mutants deficient in sulphite reductase were unable to utilize sulphonates; therefore, this enzyme must be involved in sulphonate utilization, though it is not yet known whether it acts upon the sulphonates themselves or upon the inorganic sulphite derived from them.